1. Field of the Invention
The present invention relates to a method for manufacturing metal sulfide nanocrystals using a thiol compound as a sulfur precursor, and more particularly to a method for manufacturing metal sulfide nanocrystals using a thiol compound as a sulfur precursor wherein the metal sulfide nanocrystals can have a uniform particle size at the nanometer-scale level, selective and desired crystal structures, and various shapes.
2. Description of the Related Art
Nanocrystals exhibit electrical, magnetic, optical, chemical and mechanical properties distinct from bulky materials. Since these properties are controllable depending on the size of nanocrystals, there has been a great deal of interest in nanocrystals. In particular, when compound semiconductor materials, including metal sulfides (e.g., CdS, ZnS and PbS) are formed into nanometer-sized crystals, their bandgap energies are changed due to quantum confinement effects.
Accordingly, when the structure, shape and size of nanocrystals are controlled, energy levels over a very broad range of wavelengths can be obtained while the properties of the bulky materials are varied. These compound semiconductor materials can be prepared by common vapor deposition techniques. In recent years, there have been a number of studies to prepare semiconductor nanocrystals by a wet chemistry technique wherein a precursor material is added to a coordinating organic solvent and nanocrystals are grown so as to have an intended size. According to the wet chemistry technique, as the nanocrystals are grown, the organic solvent is naturally coordinated to the surface of the nanocrystals, acting as a dispersant. Accordingly, the organic solvent allows the nanocrystals to grow to the nanometer-scale level. The wet chemistry technique has an advantage in that nanocrystals of a variety of sizes can be uniformly prepared by appropriately controlling the concentration of precursors used, the kind of organic solvents, and preparation temperature and time, etc. Since Group II-VI compound semiconductor nanocrystals can emit light in the visible region, and are easy to synthesize as compared to Group III-V compound semiconductor nanocrystals, they are actively under study.
U.S. Pat. No. 6,225,198 discloses a process for forming shaped Group II-VI compound semiconductor nanocrystals having uniform size by wet synthesis. According to this patent, the Group II-VI compound semiconductor nanocrystals are prepared by mixing a solution of a Group II element and a solution of a Group VI element in the presence of suitable dispersant and solvent under appropriate temperature conditions. Specifically, the Group II-VI compound semiconductor nanocrystals are prepared by mixing a solution of an organometallic compound containing a Group II element (e.g., dimethyl cadmium), and a solution of a Group VI element (S, Se or Te) in an organic solvent (e.g., trioctyl phosphine).
U.S. Pat. No. 6,576,291 reports a method for manufacturing Group II-VI compound semiconductor nanocrystals by mixing a solution of a Group II metal salt, such as cadmium acetate or cadmium oxide, with a solution of a compound of a Group VI element bonded with phosphine chalcogenide, bis(silyl) chalcogenide, dioxygen, ammonium salt, or tris(silyl) pnictide.
U.S. Pat. No. 6,322,901 describes core-shell structured Group II-VI and Group III-V compound semiconductor nanocrystals with improved luminescence efficiency. The core-shell structured compound semiconductor nanocrystals are prepared by forming a compound semiconductor layer on the surface of core nanocrystals. The compound semiconductor layer has a larger energy bandgap than the core nanocrystals. In addition, a method for preparing the core-shell structured compound semiconductor nanocrystals is disclosed in U.S. Pat. No. 6,207,229. According to these patent publications, the core is composed of compound semiconductors having uniform size distribution, and the compound semiconductor layer is uniformly formed on the core surface. The compound semiconductor nanocrystals thus prepared have a full width half maximum (FWHM) of 60 nm or less.
The method for manufacturing the compound semiconductor nanocrystals comprises placing the compound semiconductor core nanocrystals having uniform size distribution in a reaction solvent, and crystal-rowing a compound semiconductor precursor on the surface of the core nanocrystals at an optimum reaction temperature to form a core-shell structure (passivation process). At this time, an organometallic compound (e.g., dimethyl cadmium or diethyl zinc) is used as a metal precursor, and bis(silyl) chalcogenide-based Group VI element compound (e.g., hexamethyldisilanthiane) is used as a precursor of the Group VI element. Alternatively, Group II-VI compound semiconductor nanocrystals can be prepared by using a precursor containing both a Group II metal and a Group VI element, such as cadmium dithiocarbamate, cadmium diselenocarbamate, zinc dithiocarbamate, bis-(hexylmethyldithio) cadmium, or bis-(hexylmethyldithio) zinc, by pyrolysis (Chemistry of Material, 2001, 13, 913).
In the case where organic solvents, such as trioctyl phosphine, trioctyl phosphine oxide, trioctyl amine, hexadecyl amine and octadecene, are used to prepare Group II-VI compound semiconductor nanocrystals and core-shell structured nanocrystals by wet synthesis, precursors usable for synthesis fall within the range defined in the patent publications and journal articles mentioned above. In particular, numerous studies have focused on cadmium telluride and cadmium selenide.
On the other hand, there have been various attempts to control reaction rate by varying reaction solvents and coordination degree of precursors used to prepare metal sulfides, such as CdS, ZnS and PbS. For example, CdS nanocrystals can be prepared by using a solvent, such as octadecene, which does not coordinate to precursors and nanocrystals to be prepared, to increase the reaction rate (Angew. Chem. Int. Ed. 2002, 41, 2368). Further, ZnS, CdS and PbS nanocrystals can be prepared by using a linear amine having a small-volume alkyl group, e.g., oleylamine, as a solvent for increasing the reaction rate between precursors (J. Am. Chem. Soc., 2003, 125, 11100). Further, a great deal of research has been conducted on the preparation of metal sulfide nanocrystals by reacting a metal salt with sodium sulfide (Na2S), ammonium sulfide ((NH3)2S), or hydrogen sulfide gas in a strongly basic solution or an aqueous solution containing a surfactant, without the use of an organic solvent. Disadvantageously, the metal sulfide nanocrystals thus prepared have non-uniform size distribution and shape. For example, when the nanocrystals have an average particle size of 4 nm, they have a broad size distribution ranging from 2˜6 nm (J. Am. Chem. Soc., 1987, 109, 5649; and The Journal of Chemical Physics, 1984, 80, 4464). It has been recently found that CdS nanocrystals can be prepared by using a metal xanthate (R—CH2—CH2—O—CS2−M+) as a novel sulfur precursor under relatively mild reaction conditions (J. Am. Chem. Soc., 2003, 125, 2050).